I detect the error at checkErrors(). It's printed in my local language and translates to something like 'illigal counting'
The error sometimes generates a faul status:
glGetShaderiv(vertShader, GL_COMPILE_STATUS, &status);

I make a printf() of 'buffer' and it provides a correct visual impression of the shader-code/text.

It 'did' bypass GL_COMPILE_STATUS earlier without the error spawning a faul status, and I would have a functioning program ..

It probably takes a newbie to produce the code above ... any suggestion of a more straightforward code is welcome.